1. Field of the Invention
This invention relates to a network comprising a plurality of station units connected by a main data transmission line like a ring enabling circular transmission of communication data in digital form and adopting a master/slave method as a communication method and, in particular to a network abnormality recovery method and system capable of recovering the normal state of a network communication function as soon as possible even if the network communication function is impaired because of a broken line of the main data transmission line, an abnormality in communication means for controlling transmission and reception of communication data, or the like.
2. Description of the Related Art
Hitherto, a network generally has been known which comprises a plurality of station units connected by a ring-like data transmission line enabling circular transmission of communication data in digital form.
To exchange communication data between station units in a network configured as described above, first a communication IC for executing communication control concerning transmission and reception of communication data is provided for each station unit and communication data is sent from the communication IC in the station unit of the transmitting party in a predetermined direction on a data transmission line. The communication IC in each station unit downstream from the transmitting party on the data transmission line determines whether or not the address of the home station unit is specified as the receiving party. If the address of the home station unit is not specified as the receiving party, the communication IC sends the communication data to the station unit downstream from the home station unit over the data transmission line. On the other hand, if the address of the home station unit is specified as the receiving party, the communication IC reads the communication data over the data transmission line, thereby exchanging the communication data between the station units.
Of the networks configured as described above, hitherto a network adopting a master/slave method as a communication method has also been known. This network consists of one master station unit operating as the master and other slave station units operating as slaves.
In the network adopting the master/slave method, the master station unit performs operation management of all station units making up the network and maintains the network communication function in the normal state as a network maintaining function, and each slave station unit usually stands by in a reception wait state and upon reception of communication data containing control information for each slave station unit such as transmission start or reception start sent from the master station unit, the slave station unit suitably transmits and receives communication data including source data containing audio or video information and control data containing slave station unit control information.
However, according to the conventional network adopting the master/slave method as the communication method, if a broken line of the data transmission line occurs or an abnormal condition such as a failure occurs in the communication IC contained in a slave station unit at a midpoint position on the route between the transmitting party and the receiving party, the station units downstream from the broken data transmission line or the station unit falling into the abnormal condition cannot receive the communication data and thus are not given a transmission opportunity of communication data generated by the downstream station units. Resultantly, a problem of interfering with the information centralized management function of the master station unit for executing centralized management of control data, etc., sent from all slave station units, to be solved is involved.